One atomic mass unit is defined as exactly one-twelfth (1/12th) the mass of one atom of carbon-12.
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The carbon-12 isotope was chosen as the standard reference for measuring atomic masses.
Scientists initially determined the atomic mass unit for oxygen by taking 1/16 of the mass of an atom of naturally occurring oxygen as the unit.
Expressing atomic masses as whole numbers or close to whole numbers makes calculations and comparisons simpler and more intuitive.
Carbon was assigned an atomic mass of 1.0 u, and oxygen was assigned an atomic mass of 1.33 u.